Modulation system



Feb. 5, 1952 'A. B. BA!LEY.

MODULATION SYSTEM 3 Sheets-Sheet 1 Filed March 12, 1948 EMF-i124 Znwentqr ARNOLD B. BAILEY A. B. BAILEY MODULATION SYSTEM Feb.5, 1952 3 Sheets-Sheet 2 Filed March 12, 1948 (Ittorneg- Feb. 5, 1952 -A. B. BAILEY MODULATION SYSTEM 3 Shee'ts-Sheet 5 Filed March 12, 1948 w k MN wNQ v KOPdJDQOE Patented Feb. 5, 1952 UNITED snares ATE'NT OFFICE 2 Claims.

The present invention relates to the modulation of electrical signals. More particularly it relates to methods'of and apparatus for producing frequency modulated'radio waves.

'I-n wireless transmission systems, for example, radio broadcasting, high frequency electrical energy is'applied to a transmitting antenna and the electrical waves radiated thereby are propagated through space so that their presence may be'detected at a remote location by suitable radio receiving equipment.

In order to utilize the radio waves for the transmission of sound signals, or other informstion, it is necessary that some characteristic of the wave be modified in accordance with the waveform and intensity of the sound signals which are to be transmitted. The process of changing the characteristics of a radio wave, in accordance with the information to be transmitted, is termed modulation. For example, it is possible to modify the amplitude of a radio frequency signal in accordance with the waveform of sound waves while maintaining the frequency of the radio signal constant. This is termed amplitude modulation and is the method used by ordinary commercial broadcast stations.

In another system the amplitude is held constant and the frequency of the radio signal is varied in accordance with the sound waves which are to be transmitted. This process is termed ifr'equency modulation and is used widely in higher frequency transmission systems. Such frequency modulation transmission systems have many advantages over amplitude modulation systems. 'One important advantage is the marked reduction in the'noise level. In other words, with frequency modulation, undesirable background sounds such ascontinuous crackling or hissing, or static disturbances such as are caused by lightning, are reduced.

In practice, frequency modulation has been dimcult to obtain. For example; the center frequency, that is, the frequency about which the modulated signal varies during modulation, must be maintained constant with great precision. In addition, the extent of the frequency variation or swing produced by the modulation must be relatively large in order to obtain the best performance. These and other requirements are difficult to fulfill with the result that the transmitters now in use generally are complex and require a reat number of'vacuum tubes and associated parts. In one aspect, the present invention is concerned with simplified methods and apparatus for producing frequency modulated signals.

Accordingly, it is an object of this invention to provide improved methods and apparatus for frequency modulating a radio frequency carrier wave.

It is another object to provide simplified ap' paratus for producing such a frequency. mo.du-" lated signal.

Another object is to improve the frequency stability of such a modulation system;

Still another object of the invention is'to pro-" vide improved methods of and apparatus for the stabilization of electrical signals in apparatus subjected to varying supply voltages.

It is a further object to provide such a stab-i lized system in which the audio modulation sig-' quate frequency modulation in accordance with applied audio or control signals.

The invention, accordingly, consists in the features of construction, combinations of elements, arrangements of parts and methods of operations as will be exemplified in the struc tures and sequences and series of steps to'be hereinafter indicated and the scope of the appli-" cation of which will be set forth in the appended claims.

The above and other objects and advantages of the present invention will appear more fully from a consideration of the detailed description which follows,'taken together'with the accompanying drawings in which: Fig. 1 is a circuit diagram of a simplified frequency modulated oscillator embodying the principles of the present invention;

Fig. 2 is a modification of the circuit shown in Fig. 1; and i Fig. 3 shows, partially in block diagramform, apparatus including an additional channel for increasing the stabilization.

In order to provide a stabilized frequency modulation circuit, a quartz crystal 2' (Fig. 1) is utilized to control the center frequency and is operated in a series resonant manner. A crystal electrode l is connected by a lead 6 to a control grid 8 of a pentode vacuum tube I2; a direct current groundreturn path being provided through an adjustable resistance element M. A cathode it of this tube is connected to ground through a bias resistor l 8 which is shunted by a condenser 22. Heater 24 of this tube is connected to a suitable source of power (not shown). A suppressor grid 28 is connected internally tocathode l6. Screen grid 28 of tube I2 is connected by 18%!15 [3.2 and 34 to a suitable power source A to-a suitable power source (not shown).

(not shown). Anode 36 of this tube is connected bymeans of a lead 38 to a parallel resonant circuit comprising an inductance 42 and an adjustable capacitor 44. Positive voltage is pro vided for anode 36 from power source A through lead 34, inductance 42, and lead 38. A lead, illustrated diagrammatically by an antenna 46, is coupled directly to anode 36 by a lead 48.

The anode circuit of tube I2 is coupled also through lead 52 and condenser 54 to control grid 56 of a pentode tube 64, and through a condenser 58 to control grid 62 of a similar tube 66. Ground return paths for grids 56 and 62 are provided by resistors 68 and 12, respectively.

Cathode 14 of tube 64 is connectedto ground I through a bias resistor 16 which is shunted by a capacitor 18. Suppressor grid 82 of this tube is connected to cathode 14 by a lead 84. Cathode 86 of tube 66 is connected to ground through a bias resistor 88 which is shunted by a capacitor- 92. Suppressor grid 94 of this tube is connected "to cathode 86 by a lead 96. Heaters 98 and I62 of tubes 64 and 66, respectively, are connected Positive voltage is provided for screen grids I64 and I86 of tubes 64 and 66, respectively, from power source A through series dropping resistors I66 and H2, respectively. Screen grids I64 and I66 are coupled to power source A through radio frequency bypass condensers I I4 and I I6, respec- 1i.

tively; power source A beingcoupled to ground by a condenser II1. Anode IIB of tube 64 is connected to power source A through a load resistor I22, and anode I24 of tube 66 is coupled to power source A through a load resistor I26.

The signal appearing at anode II8 of tube 64 is coupled through a condenser I28, an inductance I32, an adjustable resistor I34, and a lead I36 to crystal electrode I38 of the crystal 2. The signal appearing at anode I24 of tube 66 is coupled through a coupling condenser I42, which is similar to condenser I28, and an adjustable capacitance I44 to lead I36.

The arrangement described above operates as a radio frequency oscillator in which the radio frequency energy is fed back from the output circuit of tubes 64 and 66 through crystal 2,, which, in this example, is series resonant near the center of the frequency range of the oscillator, to the input of tube I 2. The signals are amplified in tube I2 and the operation of crystal 2 within the correct frequency range is maintained by the parallel circuit (which is resonant at the signal frequency and includes inductance 42 and capacitance 44) .in the anode circuit of tube I2. The amplified radio frequency signal from tube I2 is divided and applied (in phase, in this example) to the control electrodes of tubes 64 and 66, respectively. The output of tube 64 is coupled through the inductance I62, which iseffectively in series with crystal 2, back to the input circuit. In a similar manner, the output from tube 66 is coupled to the input circuit through a condenser I44, which is adjusted to have a capacitive reactance numerically equal to the inductive reactance of inductance I32 at or near the center frequency of operation.

Thus, if the transconductance of tube 64 is equal to that of tube 66, equal signal currents are produced in the anode circuits because identical signals are applied to the control grid 56 and 62 of these tubes. The inductive effect of inductance I32 is compensated by the capacitive reactance effect of condenser I44.

In order to observe the effect on the circuit when the transconductances of tubes 64 and 66 are not identicalassume that tube 64 is biased to cut-01f so that no signal appears on the anode I I3 and full signal appears on anode I24 of tube 66. The condenser I44 now introduces a capacitive reactance in the only feed-back circuit which is active. This phase shift in the feedback circuit causes the circuit to generate a frequency above the center frequency.

If, however, tube 66 is biased so that it does not draw plate current and tube 64 is allowed to draw full plate current, the signal appearing at anode I I8 of tube 64 is fed back to the crystal 2 through inductance I32, thus introducing an inductive reactance in the only active feed-back circuit to cause the oscillatory circuit to generate a lower frequency.

In operation of this embodiment, neither of the tubes 64 or 66 is driven to cut-off condition, but the transconductances are caused to vary in accordance with the desired modulating signal. For example, if it is desired to modulate the radio frequency signal with audio signals, modulation voltages may be applied, in phase opposition, to tubes 64 and 66. The circuit illustrated in Fig. 1 is adapted to apply plate and screen modulation to these tubes.

The sound signals, which are to be impressed on the carrier, are translated into electrical signals by a microphone I52 and amplified in a conventional type audio amplifier I54, the output of which is coupled to a primary winding I56 of a modulation transformer I56. One end of secto the tubes 64 and 66.

ondary winding I62 of transformer I58 is coupled to anode H8 of tube 64 through condenser I64 and a radio frequency choke I66, and to screen I64 of the same tube through a condenser I68. The opposite end of secondary winding I62 is coupled to anode I24 of tube 66 through a condenser I12 and a radio frequency choke I14, and to screen I06 of this tube through a condenser I16. During modulation the audio signals are applied simultaneously, in phase opposition, to tubes 64 and 66. Thus, as the feed-back current through inductance I32 increases, because of an increase in signal current from tube 64, the feedback current through capacitance I44 decreases, because of a decrease in the signal current from tube 66, the two effects operate to lower the frequency of the generated signal; whereas during those portions of the signal when feed-back current through condenser I44 is increased, the feedback current through inductance I32 is decreased thereby increasing the frequency of the generated signal. In this manner, the frequency of the radio signal appearing on lead 48 varies in accordance with the applied audio frequency signals. For medium or low frequency operation and for narrowband frequency modulation transmission systems,

the signals appearing on lead 48 may be coupled directly to the antenna 46, or they may be amplified prior to transmission. Where higher fre- 'quency operation with correspondingly greater frequency swing is desired, conventional multipliers may be utilized as necessary.

It is to be noted that modulation signals to be effective must be applied in phase opposition This characteristic is important from the standpoint of frequency stability; thus, if the plate and the screen voltages of tubes 64 and 66 are increased or decreased simultaneously, the center frequency remains relatively constant because of the compensating eifects of inductance I32 and capacitance I44.

and because-of the inherent voltagel'stability or the circuit as a whole.

The index: of modulation, thatv issithe extentof the.- frequencyswing with: a:. given audio.- signal, dependsupon: the magnitudes of the reactances of condenser Md and inductance I32 and. upon: thez-frequencyrofs operation. The characteristics of the: crystal and: oi? its electrodes. also affect. the extendof. the frequency: swing: which. may be: obtained;

The: limitation orr frequency. swing; caused. by the. nearnessof the; parallel. resonant frequency. (antieresonantitrsquencyy ofr the: crystal to. its series: resonant: frequency may be: overcome. sub? stantially by. placing; a paralleli circuit; comprisiing a; condenser like and air inductance I182, in. shunt: with: theecrystal the=combinedzcrystal .cir-- cuit; being: resonant at' -the center operating frequencya It-is apparent that the presentinventionhas utility fOIT' INHZpOSESE other: than: transmission, of

audio signals. For example,"thetransconduca ancea-oi tubes A- and 5.6 may be controlled by the application of direct control voltages to control grids 62 and 56, respectively; so that the generatedsignalmay be. caused to-vary in accordance with these DC control potentials. Because of the stabilization. characteristics of the tubes and circuit arrangement and because modulation control; is readily applied, the arrangement is well suited for use in multiplex transmission. syst'eins which the. carrier is shifted a. given amount. by a keying action in; accordance with the: applied signals.

Suitable: values. for use. the arrangement shown in. Fig. L areflgiven'inthefollowing table:

Tubes Number Type Resistance elements Value In Thousands of Ohms Number Capacitance elements Value. In

N m Micrcfarads 0. oooozt-DI 002 0. 0000254). 002

coo

@UBQASQ 6 Thequartz: crystal 2', for example in this: embodiment, may be resonant-at abouts megacyclesand is a conventional type such as are used ordinarily for frequency control'purposes; However, operation may be improved for these special: applications by judicious .choice of electrode sizes, shapes, and placement, and by loading the surfaces of the crystal.

Fig. 2 shows a. slightly different: embodiment of thecircuits shown inFi'g. r and whichlis adapted toybe: coupled to: conventional .equipmentcfor' multiplying and;- amplifying the signal produced" by the oscillator so thatv the-radiated. signal may have the desiredcenter frequency and extentioi modulation;

An output coil: 2532; is coupled: to inductance 42a in the anode circuit of. tube IZaandi iscoupled through leads 2M and: 206 to the desired multiplying: or amplifying; circuits (notshown);

. In thisv example, the system for modulating;

connection 2538 of. winding Idea" of: transformer Iiiia. The basic operation; howeven. of thecibcui-tremainsthe same as that described; in connection with. Fig. 1.

It is apparent, that many changes and modifications may be made. .in, theabove described.

system, in order toadapt it to various functions.

and applications, without. departing from the.

scope or spirit of theinventionu For. example, other elements may be substituted for thecrystal- 2 andendlessvarieties ofmodulation circuitsand methods. of application -maybe devised. The.

resonant circuit comprising inductance 42 and condenser M may be replaced by a pre-tuned band pass filter having any desired number of sections and characteristics.

Fig. 3 shows another modulated signal generator in which the feed-back current comprises three components: a first component from modulator circuit 299 (which is similar to tube 66 with its associated circuits) through phase shift condenser lMb; a second component from modulator circuit 2 I 2 (similar to tube 64 with its associated circuits) through phase shift inductance l32b; and a third component in which no modulation is effected and which provides a stabilizing influence on the circuit.

The signal from tube l2b and the associated components, which are similar to the corresponding components and tube I2 of Fig. 1, is divided at point 214 and applied to modulator circuits 209 and M2, and through condenser 2|6 to control grid 21s of tube 222. A ground return for grid 2 i8 is provided by a resistor 224. Suppressor grid 226 of this tube is connected to ground through bias resistor 232 which is in shunt with a condenser 234.

Screen grid 235 of tube 222 is connected to power source A through a series resistance 238..

which is in parallel with a condenser 242. Anode 244 is connected to power source A through a load resistor 246.

No modulation signal is applied to tube 222 so that the circuit amounts to a straight through amplifier the output signals from The output signals from these circuits are comblned with the signals from tube 222 at point 256. The resonant circuit 254 may comprise a crystal arrangement such as is shown in Fig. v1 or it may comprise other forms of resonant circuits such as combinations of inductances and capacitances, bridge circuits, magnetostrictive coupling arrangements and so forth.

The feed-back signal from circuit 254 is applied to the input circuit of tube I25 to thereby generate a signal having a frequency dependent upon the relative instantaneous magnitudes of the three components of the feed-back signal.

As many possible embodiments maybe made of the above invention, and as many changes might be made in the embodiment above set forth, it is to be understood that all matter hereinbefore set forth or shown in the accompanying drawing is to be interpreted as illustrative and not in a limiting sense.

t 'I claim:

1. In a transmission system, the method of producing a frequency modulated signal comprising the steps of amplifying a first signal, dividing said signal into at least two components, oppositely shifting the instantaneous phase of at least two of said components relative to each other, varying the magnitude of the shifted components in accordance with an applied modulation signal, passing said components along a common path, difierentially impeding the flow of said components along said common path, the impeding being least substantially at the center of the frequency range covered by said signal components, and combining the resulting signal with said'first reamplifying it therewith.

2. Ina frequency modulation system, a main amplifier having output and inputcircuits, a first and a second feed-back amplifier each hav'-- ing an input and .an output circuit, said inputcircuits of said feed-back amplifiers being coupled' to said output circuit of said main amplifier, a series circuit resonant within the operating frequency range and coupled to the input circuit of said main amplifier and to the output circuit of said first feed-back amplifier through a capacitive element and to the output circuit of said second feed-back amplifier through an inductive element, said inductive and capacitive elements having substantially equal reactance values at one frequency within the operating frequency range, and a source of control voltage coupled to said feed-back am plifiers for controlling the instantaneous signal output thereof in. accordance with the magnitudeof said control voltage. .J-

' ARNOLD B. BAILEY.

signal and I REFERENCES CITED e The following'references are of record in the file of this patent:

UNITED STATES PATENTS Usselman Mar. 8, 

